bug#31744: 26.1; Improvements to make tags and make -C test

[Noam Postavsky <npostavs@gmail.com>]

>>> * src/Makefile.in: Create TAGS files in ${srcdir}, not build dir.
>>
>> I'm not sure I agree with this.  Other projects I looked at produce
>> TAGS in the build directory (but reference source files in srcdir, of
>> course).  Why do you think TAGS should go to the source directory?

Oh, hmm.  The original reason, is that when looking at a source file in
Emacs, and then hitting M-. I get a prompt to visit the TAGS table,
which starts from the source directory.  Then I have to go looking for
the TAGS file in the corresponding build directory.  This seems kind of
silly, especially since every build directory will have identical TAGS
files anyway (meaning I either have to make TAGS redundantly per-build
directory, or remember exactly which build directory has the TAGS file).

I saw the FIXME in Makefile.in, so I assumed (oops!) this was some
well-known problem, and that nobody had bothered to make the obvious fix
yet.

> They should not. Source should be able to be mounted read-only, and
> all build products should be stored elsewhere.

Emacs puts .elc files in the source directory.  I was under the
impression that the general principal is that arch & config dependent
files go in the build directory, and arch & config independent ones go
in the source directory.